Liquid developers for electrostatic photography

ABSTRACT

An improved liquid developer composition for converting a latent electrostatic image into a visible image and for fixing the visible image into a permanent visual record, consists essentially of carrier liquid comprising e.g. trichlorotrifluoroethane or carbon tetrachloride and having a high electrical resistance, 0.005 to 0.1 percent by weight of of finely divided pigment powder (Fe3O4, TiO2, graphite) dispersed or suspended in said liquid, 0.001 to 0.3 percent by weight of metal naphthenate (cobalt naphthenate, nickel naphthenate, manganese naphthenate) which dissolves in said liquid and which comprises 4 to 10 percent by weight of metal ions, and 0 to 1 percent by weight of insulating oil.

United States Patent Otsuka et al.

[ 1 Feb.1,1972

[54] LIQUID DEVELOPERS FOR ELECTROSTATIC PHOTOGRAPHY [72] Inventors: Tetsuro Otsuka, Takatsuki-shi; lsao Ota, Osaka-shi; Shinichi Ogo, Moriguchi-shi, all

[211 App1.No.: 747,020

Related US. Application Data [63] Continuation of Ser. No. 526,713, Feb. 11, 1966,

abandoned.

[30] Foreign Application Priority Data June 1, 1965 Japan ..40/33025 June 30, 1965 Japan ..40/39868 [52] US. Cl ..252/62.1 [51] lnt.Cl. .....G03g 9/04 [58] Field of Search ..252/62.l

[56] References Cited UNITED STATES PATENTS 3,241,957 3/1966 Fauseretal. ..252/62.1

Metcalfe et a1 ..252/62.l Metcalfe et a1. ....252/62.l Beyer ....252/62.l Wagner ..252/62.l

Primary Examiner-George F. Lesmes Assistant Examiner-J. P. Brammer [5 7] ABSTRACT An improved liquid developer composition for converting a latent electrostatic image into a visible image and for fixing the visible image into a permanent visual record, consists essentially of carrier liquid comprising e.g. trichlorotrifluoroethane or carbon tetrachloride and having a high electrical resistance, 0.005 to 0.1 percent by weight of of timely divided pigment powder F6 0,, TiO graphite) dispersed or suspended in said liquid, 0.001 to 0.3 percent by weight of metal naphthenate (cobalt naphthenate, nickel naphthenate, manganese naphthenate) which dissolves in said liquid and which comprises 4 to 10 percent by weight of metal ions, and 0 to 1 percent by weight of insulating oil.

4 Claims, No Drawings This application is a continuation of Ser. No. 526,713 filed Feb. 1 l, 1966 and now abandoned.

This invention relates to electrostatic printing. More par ticularly it relates to an improved liquid developer composition for converting a latent electrostatic image into a visible image and for fixing the visual image into a pennanent visual record.

It is well known that a latent electrostatic image formed on an insulating photoconductive material can be immediately converted into a visible image by applying an electrically charged pigment which is dispersed in a liquid vehicle having a high electrical resistance and can be fixed into a permanent visual image by a per se known procedure. The literature discloses many modifications of the developing process as well as developer compositions. The prior art developers, however, are not entirely satisfactory from the viewpoint of producing a fine visual image which has fine granularity, high resolution and continuous tone, etc.

The following are required for obtaining a fine permanent visual image in a short development time:

1. The pigment in finely divided powder form exists in a finely dispersed state in a liquid vehicle;

2. All the powder particles acquire an electrostatic charge of one sign in the vehicle;

3. The vehicle is a highly volatile liquid;

4. A visible image can be fixed immediately when the liquid is evaporated.

It is an object of this invention to provide a liquid developer composition which is able to produce a fine visual permanent image in a short period of development time.

It is another object of this invention to provide a liquid developer composition characterized by pigment powder dispersed finely and charged with the same polarity in a liquid vehicle, and by liquid vehicle having a high vapor pressure to enable an electrophotographic layer to be dried quickly.

It is a further object of this invention to provide a liquid developer which is useful for a transparent electrophotographic film.

It is a still further object of this invention to provide a process of electrostatic printing, which is able to convert a latent electrostatic image into a visible image and simultaneously to fix the visible image using a liquid developer of the present invention.

According to the present invention, a novel liquid developer composition consists essentially of a liquid vehicle, finely dispersed pigment powder, and an organometallic compound which makes it possible to provide the pigment powder with the same charge and to disperse the powder homogeneously in the vehicle. A liquid vehicle is usually called a carrier liquid and is required to have a high electrical resistance more than ohm-cm. to be relatively nontoxic and noninflammable, and to be highly volatile for quick drying. It is also necessary that a carrier liquid does not react with ingredients of the developer composition and does not solve constituent materials of an electrophotographic layer having a latent electrostatic image thereon. Although numerous carrier liquids have been disclosed where the constituent material of the layer is an inorganic photoconductive material, such as selenium or zinc oxide, it is difficult to find a carrier liquid suitable for an organic photoconductive material such as poly-N-vinylcarbazole sensitized by an activator or a dyestuff compound. According to the present invention, it has been discovered that a carrier liquid comprising trichloro-trifluoroethane or carbon tetrachloride does not solve the constituent materials of an organic photoconductive layer and satisfies the aforesaid requirements.

Pigment powder can be formed of any fine powder which is inert to the liquid developer constituents and is in a particle size less than 1 micron. The finer powder has a tendency to coagulate with a passage of time when the powder is dispersed in the carrier liquid. It is important for obtaining an entirely satisfactory visible image that the finer powder be finely dispersed in the carrier liquid for a long time. Although carbon black of prior art developers is known as a fine pigment powder, it agglomerates easily when suspended into a carrier liquid. In addition, the carbon black in the liquid is also difficult to be charged electrostatically in the same sign. This may be due to an irregular surface state of the carbon black.

It has been discovered according to the present invention that pigment powder such as phthalocyanine blue, magnetite, graphite, zinc oxide, magnesium oxide, or titanium dioxide in a particle size less than 1 micron, respectively, is hard to coagulate for a long time period of suspension in a carrier liquid. Although it has been disclosed that electrically conducting powder such as carbon black destroys a latent image, graphite powder can be employed as an excellent pigment powder and produces an entirely satisfactory visible image in accordance with the present invention. Since the graphite powder has a flaky shape in a small dimension, it is easily adhesive to the surface of a photoconductive layer when the carrier liquid evaporates off. It is more preferable to use colloidal graphite commercially available as an oildag" consisting of 10 percent by weight of fine graphite powder and percent by weight of petroleum oil (Acheson Colloids Ltd., Prince Rock, Plymouth, Devon, England).

Although white titanium dioxide powder is not desirable as the pigment for producing a fine visual image on an opaque electrophotographic plate or paper, it gives an entirely satisfactory visible image when the visual image produced on a transparent electrophotographic film comprising a transparent organic photoconductive material such as poly-N- vinylcarbazole is projected on a screen. This is thought to be due to the great reflective property of titanium dioxide powder. As mentioned above, titanium dioxide powder is not suitable for an opaque photoconductive layer but it is preferable to employ a mixture of titanium dioxide and graphite in finely divided powder in order to recognize the developed image prior to projection.

Magnetite powder or titanium dioxide powder may be commercially available powder prepared by a well-known chemical technique.

Since a high concentration of pigment powder in a carrier liquid has a tendency to promote the agglomeration of pigment powder and to elevate the viscosity of the carrier liquid, the electrophotographic layer withdrawn from the liquid developer may be accompanied by undesirable deposition of excess pigment powder due to its high concentration in the liquid, causing impairment of the visual image. In addition, it has been also disclosed that a low concentration results in a slow rate of a developing process. Prior patents have disclosed that a preferable concentration of pigment powder is l to 25 percent by weight for a liquid developer composition, although the ingredients of prior art developers differ clearly from those of the new composition of the present invention. According to the present invention, the following percentages by weight of pigment powder can be employed: Operable amounts of pigment are 0.005 to 0.l percent by weight and preferable amounts are 0.0l to 0.05 percent by weight. Such a low concentration of pigment powder can produce sufficiently a fine visible image in a development time of not more than several seconds.

As already mentioned, it is necessary for producing an entirely satisfactory visible image that all the powder particles be charged with the same polarity; otherwise the visible image developed by the developer does not reproduce an original, that is, the background has a fog and the visual image has double black borders. The mechanism of the electrostatic charging process of dispersed powder in a carrier liquid is not well understood. It has been discovered according to this invention that an addition of metallic soap to a carrier liquid is useful for producing uniformity of the electrostatic charges on the powder particles, and for increasing the amount of their positive charges, which results in a satisfactory visible image. It is important that the metallic soap be soluble in the carrier liquid and that the electrical resistance of the carrier liquid be not appreciably decreased by its addition. Operable metallic soaps are fatty soaps, resin soaps and naphthenate soaps comprising metal ions other than alkali metal ions.

Electrophoresis tests can determine the percentage of the number of pigment particles charged in the same sign, for example, positively. When the pigment particles dispersed in the carrier liquid are subjected to electrophoresis, the particles charged plus" move to the cathode and the particles charged "minus to the anode. if all particles are charged, for example, minus," all particles move to the anode and no particles move to the cathode. By this method one can determine the polarity of the electrical charges of pigment particles. The addition of metallic soap clearly elevates the percentage of pigment particles charged in the same sign.

Especially an addition of cobalt naphthenate, nickel naphthenate or manganese naphthenate to a carrier liquid comprising trichloro-trifluoroethane or carbon tetrachloride gives an entirely satisfactory visible image. An increase in the additive amount of metal naphthenate results in a low electrical resistance of carrier liquid. This is undesirable because of the discharge of a latent electrostatic image. Therefore, the maximum amount of metal naphthenate must be controlled from the standpoint of the electrical resistance of carrier liquid. The minimum amount of metal naphthenate can be determined by the amount necessary to produce pigment powder charged entirely in the same sign. According to this invention, preferable amounts of metal naphthenate are 0.001 to 0.3 percent by weight. It is a great feature of this invention that the uniformity of electrical charges on the powder particles can be achieved by the addition of 0.001 to 0.3 percent by weight of metal naphthenate selected from the group consisting of cobalt naphthenate, nickel naphthenate and manganese naphthenate where the pigment powder is in a particle size less than 1 micron and the carrier liquid has a high electrical resistance over 10 ohm-cm.

Metal naphthenate is represented by the chemical formula where n is a variable and M is a divalent metal. Metal naphthenate can be prepared by the reaction of an aqueous solution of metal acetate such as cobalt acetate, nickel acetate or manganese acetate with a basic aqueous solution naphthenic reaction of an aqueous solution of metal nitrate such as cobalt nitrate, nickel nitrate or manganese nitrate with a benzene solution of naphthenic acid; or by heating a mixture of naphthenic acid and metal oxide such as cobalt oxide, nickel oxide or manganese oxide. Since the naphthenic acid is a product of petroleum distillation, it is difficult to exactly define the n. Actual naphthenic acid comprises materials of various n values. An increase in It results in a decrease in a weight percent of metal in the total molecular weight of the naphthenate.

According to this invention, a higher weight percentage of metal is preferable for producing a uniformity of electrical charges on powder particles when dispersed in a carrier liquid. A preferable metal naphthenate composition comprises 4 to percent by weight of metal such as cobalt, nickel or manganese, and a more preferable metal naphthenate is the solid metal naphthenate comprising 8 percent by weight of metal. From these facts, it is preferable that the naphthenate compound comprising 4 to 10 percent by weight of cobalt, nickel or manganese be added to a carrier liquid in an amount of 0.001 to 0.3 percent by weight. Furthermore, the addition of metal naphthenate to the carrier liquid causes the following effect: The dispersion of the pigment powder in the carrier liquid becomes more stable and homogeneous, and accordingly satisfactory visible images can be rapidly developed even in the low concentration of the pigment powder according to this invention.

The dispersion of pigment powders in carrier liquid can also be maintained satisfactorily for a long time by adding a small amount of oil into the dispersion system. Any oil having a high electrical resistance can be employed for the above-mentioned purpose, for example, soya bean oil, linseed oil, mineral oil, olive oil, silicone oil, chlorinated diphenyl, etc. However, a preferable amount of oil is less than 1 percent by weight because a large amount of oil results in a slow drying of the photoconductive layer and impairs the visual image by the oil remaining as smears on the layer.

A visible image can be fixed immediately by a fixing agent contained in the carrier liquid. It is required that the fixing agent dissolve in the carrier liquid and that it cure easily when the carrier liquid evaporates off. In addition, it is important that an addition of a fixing agent does not lead to a decrease in electrical resistance of the carrier liquid and does not infiuence the polarity of the electrical charges of the pigment powder particles. Due to solubility characteristics of fixing agent, the latter varies with the carrier liquid. Any resin or rosin satisfying the above requirements can be employed. A preferable fixing agent is chlorinated triphenyl for a carrier liquid of trichloro-trifluoroethane, and chlorinated triphenyl or polystyrene for a carrier liquid of carbon tetrachloride. The amount of additive fixing agent must be controlled from the standpoint of avoiding the impairment of the image when the electrophotographic layer is withdrawn from the liquid developer. The available amount of fixing agent is 0.05 to 0.5 percent by weight of chlorinated triphenyl or polystyrene where the carrier liquid is trichloro-trifiuoroethane or carbon tetrachloride.

According to the present invention, the preferable liquid developer composition consists essentially of a carrier liquid such as trichloro-trifluoroethane or carbon tetrachloride having a high electrical resistance, 0.005 to 0.1 percent by weight of finely divided pigment powder selected from the group consisting of magnetite, titanium dioxide and graphite, 0.001 to 0.3 percent by weight of a metal naphthenate selected from the group consisting of cobalt naphthenate, nickel naphthenate and manganese naphthenate, 0 to 1 percent by weight of an insulating oil and 0.05 to 0.5 percent by weight of a fixing agent such as chlorinated triphenyl or polystyrene dissolved in said carrier liquid, said pigment powder being in a particle size less than 1 micron and being finely dispersed in said carrier liquid.

A liquid developer of the present invention can be prepared as follows: A mixture of a small amount of carrier liquid, pigment powder such as magnetite, titanium dioxide or graphite in a particle size less than 1 micron, metallic soap such as metal naphthenate and oil in a given composition is ball-milled for about 10 hours to form a homogeneous paste. The paste so produced is added to the carrier liquid such as trichlorotrifluoroethane or carbon tetrachloride in a given weight ratio and stirred by means of an ultrasonic vibrator to obtain a well dispersed liquid developer. A fixing agent such as chlorinated triphenyl or polystyrene is added to the carrier liquid prior to the ultrasonic vibration.

The novel liquid developer of the present invention can be applied to a latent electrostatic image produced by any of the known procedures recognized in the prior art. For example, uniform positive or negative charges are imparted to any electrophotographic plate, paper or film by any of the known means such as corona discharge. Then, the electrophotographic layer is exposed to a pattern of light and shadow illumination whereby the illuminated areas are discharged and a charge image pattern remains on the layer which is developed developer composition, quick drying can be achieved and a fine visible image can be readily obtained. The fine visible image can be simultaneously fixed, by a fixing agent dissolved in the carrier liquid, into a permanent record when the layer is dried. By using the liquid developers of the present invention, satisfactorily fine permanent visual images can be obtained .even where a long electrophotographic film is required to be continuously developed for 3 seconds at a developing rate of 12 cm. per second.

The following examples of specific new compositions are given by way of illustration and should not be construed as limitative. In these examples, g." signifies gram(s) and ml." signifies milliliter(s).

EXAMPLE 1 Ten grams of phthalocyanine blue is suspended in 2.00 ml. of carbon tetrachloride and the suspension is milled for about hours in a ball mill. Five milliliters of the phthalocyanine blue suspension is diluted by 500 ml. of carbon tetrachloride, in which there is dissolved 0.2 g. of cobalt naphthenate containing 8 percent by weight of cobalt metal ions. The resulting dispersion is a liquid developer which selectively deposits on a negatively charged latent electrostatic image.

The amount of cobalt naphthenate available is in a range of 0.02 to 2 g. for 500 ml. of carbon tetrachloride. Any cobalt naphthenate comprising 4 to l0 percent by weight percent ages of cobalt metal ions can lead to the same result.

When any electrophotographic layer carrying a negatively charged latent electrostatic image is submerged in the resulting liquid developer for a few seconds, a blue visible image of the pattern is produced on the layer and it is a direct (or positive) print of the original. When it is necessary to fix permanently the visible image on the layer, 1 g. of polystyrene is additively dissolved in 500 ml. of the developer. In this case the visual image is sufficiently fixed simultaneously when the carrier liquid is evaporated. Polystyrene is useful as a fixing agent for a carrier liquid such as carbon tetrachloride.

EXAMPLE 2 One hundred and fifty milligrams of cobalt naphthenate having 8 percent by weight of cobalt metal ions is dissolved in 500 ml. of trichloro-trifluoroethane CCl F-CClF Two hundred milligrams of titanium dioxide powder having a particle size in the range of 0.2 to 0.4 microns is completely dispersed in the aforesaid carrier liquid by means of an ultrasonic vibrator. The dispersion is stable for a long time when it has incorporatcd therein a small amount of oil, e.g., olive oil, linseed oil or chlorinated diphenyl in the range of 0.0l to 0.l percent by weight.

The visible image is developed in a similar way to that of Example l. A fine white visible image is obtained when this liquid developer is applied to an opaque electrophotographic plate or paper carrying a latent electrostatic image. The image in white color is not suitable for an opaque photoconductive layer. However, the resultant developer is a useful liquid developer for developing a negatively charged latent electrostatic image on a transparent photoconductive layer and for projecting the obtained visual pattern onto a screen. The transparent electrophotographic film is produced by coating a transparent base film having a transparent electrically con ducting layer (e.g., Cul layer) with a transparent organic photoconductor such as those described in the Hoegl et al. US. Pat. No. 3,037,861, issued June 5, 1962. The image produced on the aforesaid film by this developer gives a satisfactory black visible pattern when enlargedly projected on a screen.

EXAMPLE 3 Fifty grams of finely divided black magnetite (Fe O powder, lOO ml. (milliliters) of linseed oil and 200 ml. of trichloro-trifluoroethane are blended in a ball mill to form a homogeneous paste. Five grams of the paste so produced is dispersed in 1 liter of trichloro-trifluoroethane which contains in solution 0.4 g. (gram) of manganese naphthenate with 8 percent by weight of manganese metal ions.

When the resulting liquid developer is applied to an opaque or a transparent photoconductive layer bearing a latent electrostatic image in a similar way to that described in Example 1, a black visible image is produced successfully on the layer.

There are two possible methods to obtain the visible image permanently fixed on a photoconductive layer. One is the method of again submerging the photoconductive layer bearing the visible image into liquid carbon tetrachloride containing in solution only 0.1 to 0.2 percent by weight of polystyrene. This method has the defect that the development and the fixing involve two different steps. The other developer is prepared by dissolving additively 450 mg. (milligrams) of chlorinated triphenyl in 300 ml. of the above obtained developer. Employment of this developer produces a permanently fixed visible image on the photoconductive layer simultaneously with developing and drying of the layer.

EXAMPLE 4 Thirty grams of finely divided graphite powder having an average particle size of 0.5 microns, 30 g. of nickel naphthenate involving 8 percent by weight of nickel metal ions and 100 g. of oil are mixed in 200 ml. of trichlorotrifluoroethane and then blended in a ball mill to obtain a homogeneous paste.

When 1 g. of the paste so produced is dispersed in 300 ml. of trichloro-trifluoroethane, a positively charged liquid developer is obtained. As in Example 1, a latent electrostatic image is rendered visible in the development time of several seconds. The electrophotographic layer can comprise either an opaque inorganic or a transparent organic photoconductive material. The visible image developed by the developer is finely black.

The visible image on the photoconductive layer can be immediately fixed by adding 0.3 g. of chlorinated triphenyl in 300 ml. of the resultant developer. Chlorinated triphenyl is an effective resinous fixing agent soluble in a carrier liquid, especially when the carrier liquid is trichloro-trifluoroethane and the pigment powder is finely divided graphite particles.

EXAMPLE 5 A liquid developer is prepared by mixing 2 g. of an oildag" with 1 liter of trichloro-trifluoroethane containing in solution 0.5 g. of cobalt naphthenate and 2 g. of chlorinated triphenyl. The resulting developer is a very stable graphite dispersion and no settling or agglomeration of the graphite particles takes place for a great many days.

The image is developed in a similar way to that of Example 1. The developer so prepared has excellent developing quality and moreover the preparation of this developer is a very simple procedure. The visible image produced by the developer has high quality, both on continuous tone and on line copy. Although the pigment concentration is very low, the development time is less than several seconds. When a transparent photoconductive layer having a latent electrostatic image is submerged in the liquid developer for 3 seconds, the maximum optical density of the obtained visible image is more than 2, and the developer makes it possible to obtain a resolution of more than 50 line pairs per millimeter on line copy.

The visible image is fixed simultaneously when the electrophotographic layer is withdrawn from the developer.

EXAMPLE 6 Two hundred milliliters of trichloro-trifiuoroethane, I00 g. of oildag" consisting of 10 percent by weight of fine graphite powder and percent by weight of petroleum oil, 50 g. of finely divided titanium dioxide powder, and 20 g. of cobalt naphthenate involving 8 percent by weight of cobalt metal ions are milled together in a ball mill until blending is complete. Two grams of the paste so produced is completely dispersed in 1 liter of trichloro-trifluoroethane, in which liquid is dissolved 0.2 g. of cobalt naphthenate. The resulting dispersion is a liquid developer which selectively deposits on negatively charged areas of an electrophotographic layer.

A gray visible image is obtained when an electrophotographic layer having a latent electrostatic image thereon is developed by this developer. Therefore, if this developer is applied to the transparent electrophotographic film stated previously in Example 2, the obtained visible image can be clearly recognized prior to projection and the finely black pattern is reproduced on a screen when enlargedly projected.

What is claimed is:

1. A composition of positively charged liquid developer for converting a latent electrostatic image into a visible image and for fixing the visible image into a permanent visual record on a transparent electrophotographic layer comprising transparent organic photoconductive materials consisting essentially of:

1 Carrier liquid consisting essentially of trichloro- 2. 0.005 to 0.1 percent by weight of finely divided pigment powder dispersed or suspended in said carrier liquid,

3. 0.00l to 0.3 percent by weight of metal naphthenate which dissolves in said carrier liquid and comprises 4 to 10 percent by weight of metal selected from the group consisting of cobalt, nickel and manganese, said metal naphthenate acting as a charge control agent for causing said pigment powder to be charged uniformly in positive polarity, and

4. 0.05 to 0.5 percent by weight of chlorinated triphenyl which dissolves in said carrier liquid and acts as a fixing agent.

2. A composition according to claim 1 wherein the pigment powder is selected from the group consisting of phthalocyanine blue, magnetite, titanium dioxide and graphite. said pigment powder being in a particle size less than 1 micron.

3. A composition according to claim 1 wherein the carrier liquid is trichloro-trifluoroethane and the pigment powder is graphite, said graphite being finely divided powder in a particle size less than 1 micron.

4. A composition according to claim 1 wherein the carrier liquid comprises less than 1 percent by weight of insulating oil selected from the group consisting of soya bean oil, olive oil, linseed oil, silicone oil, chlorinated diphenyl and petroleum oil. 

2. 0.005 to 0.1 percent by weight of finely divided pigment powder dispersed or suspended in said carrier liquid,
 2. A composition according to claim 1 wherein the pigment powder is selected from the group consisting of phthalocyanine blue, magnetite, titanium dioxide and graphite, said pigment powder being in a particle size less than 1 micron.
 3. A composition according to claim 1 wherein the carrier liquid is trichloro-trifluoroethane and the pigment powder is graphite, said graphite being finely divided powder in a particle size less than 1 micron.
 3. 0.001 to 0.3 percent by weight of metal naphthenate which dissolves in said carrier liquid and comprises 4 to 10 percent by weight of metal selected from the group consisting of cobalt, nickel and manganese, said metAl naphthenate acting as a charge control agent for causing said pigment powder to be charged uniformly in positive polarity, and
 4. 0.05 to 0.5 percent by weight of chlorinated triphenyl which dissolves in said carrier liquid and acts as a fixing agent.
 4. A composition according to claim 1 wherein the carrier liquid comprises less than 1 percent by weight of insulating oil selected from the group consisting of soya bean oil, olive oil, linseed oil, silicone oil, chlorinated diphenyl and petroleum oil. 